1. Field of the Invention
The present invention relates to the papermaking arts. More specifically, the present invention relates to through-air-drying (TAD) fabrics used in the manufacture of bulk tissue and towel, and of nonwoven articles and fabrics.
2. Description of the Prior Art
Soft, absorbent disposable paper products, such as facial tissue, bath tissue and paper toweling, are a pervasive feature of contemporary life in modern industrialized societies. While there are numerous methods for manufacturing such products, in general terms, their manufacture begins with the formation of a cellulosic fibrous web in the forming section of a paper machine. The cellulosic fibrous web is formed by depositing a fibrous slurry, that is, an aqueous dispersion of cellulose fibers, onto a moving forming fabric in the forming section. A large amount of water is drained from the slurry through the forming fabric, leaving the cellulosic fibrous web on the surface of the forming fabric.
The cellulosic fibrous web is then transferred to a through-air-drying (TAD) fabric or belt by means of an air flow, brought about by vacuum or suction, which deflects the web and forces it to conform, at least in part, to the topography of the TAD fabric or belt. Downstream from the transfer point, the web, carried on the TAD fabric or belt, passes through a through-air dryer, where a flow of heated air, directed against the web and through the TAD fabric or belt, dries the web to a desired degree. Finally, downstream from the through-air dryer, the web may be adhered to the surface of a Yankee dryer and imprinted thereon by the surface of the TAD fabric or belt, for further and complete drying. The fully dried web is then removed from the surface of the Yankee dryer with a doctor blade, which foreshortens or crepes the web and increases its bulk. The foreshortened web is then wound onto rolls for subsequent processing, including packaging into a form suitable for shipment to and purchase by consumers.
As noted above, there are many methods for manufacturing bulk tissue products, and the foregoing description should be understood to be an outline of the general steps shared by some of the methods. For example, the use of a Yankee dryer is not always required, as, in a given situation, foreshortening may not be desired, or other means, such as “wet creping”, may have already been taken to foreshorten the web.
It should be appreciated that TAD fabrics may take the form of endless loops on the paper machine and function in the manner of conveyors. It should further be appreciated that paper manufacture is a continuous process which proceeds at considerable speeds. That is to say, the fibrous slurry is continuously deposited onto the forming fabric in the forming section, while a newly manufactured paper sheet is continuously wound onto rolls after it is dried.
Those skilled in the art will appreciate that fabrics are created by weaving, and have a weave pattern which repeats in both the warp or machine direction (MD) and the weft or cross-machine direction (CD). Woven fabrics take many different forms. For example, they may be woven endless, or flat woven and subsequently rendered into endless form with a seam. It will also be appreciated that the resulting fabric must be uniform in appearance; that is, there are no abrupt changes in the weave pattern to result in undesirable characteristics in the formed paper sheet. In addition, any pattern marking imparted to the formed tissue will impact the characteristics of the paper.
Contemporary papermaking fabrics are produced in a wide variety of styles designed to meet the requirements of the paper machines on which they are installed for the paper grades being manufactured. Generally, they comprise a base fabric woven from monofilament and may be single-layered or multi-layered. The yarns are typically extruded from any one of several synthetic polymeric resins, such as polyamide and polyester resins, used for this purpose by those of ordinary skill in the paper machine clothing arts.
The present application is concerned, at least in part, with the TAD fabrics or belts used on the through-air dryer of a bulk tissue machine although it may have other applications beyond this. However, the present application is primarily concerned with a TAD fabric.
such fabric may also have application in the forming section of a bulk tissue or towel machine to form cellulosic fibrous webs having discrete regions of relatively low basis weight in a continuous background of relatively high basis weight. Fabrics of this kind may also be used to manufacture nonwoven articles and fabrics, which have discrete regions in which the density of fibers is less than that in adjacent regions whereby the topography of the nonwoven article is changed, by processes such as hydroentanglement.
The properties of absorbency, strength, softness, and aesthetic appearance are important for many products when used for their intended purpose, particularly when the fibrous cellulosic products are facial or toilet tissue, paper towels, sanitary napkins or diapers.
Bulk, cross directional tensile, absorbency, and softness are particularly important characteristics when producing sheets of tissue, napkin, and towel paper. To produce a paper product having these characteristics, a fabric will often be constructed so that the top surface exhibits topographical variations. These topographical variations are often measured as plane differences between strands in the surface of the fabric. For example, a plane difference is typically measured as the difference in height between a raised weft or warp yarn strand or as the difference in height between MD knuckles and CD knuckles in the plane of the fabric's surface. Often, the fabric surface will exhibit pockets in which case plane differences may be measured as a pocket depth.
Additionally, drying capability of an industrial fabric is very essential for its use in processes such as TAD. Typically, a standard TAD fabric design in the papermaking industry for making paper towel, which is a 5-shed, 3×2 weave pattern. This design exhibits higher sheet caliper and absorbency, which allows lower sheet basis weight. The other design that is typically used in toilet tissue production is a 5-shed, 4×1 weave pattern which has demonstrated to result in a higher sheet softness. Both designs have proven to be robust in the hot, humid, TAD environment with better sheet properties. Fabric designers realize that pocket depth formed by the weave pattern is also important so multilayer thicker fabrics have been tried. However, these multilayer designs pose some serious drawbacks, such as increased fabric water content as they generally carry more water, which results in higher drying time. The primary mechanism for producing low density high caliper tissue webs with the TAD process is the pocket depth of the fabric. Therefore, it is the pocket depth of the fabric that dictates the caliper of the tissue web. A close study of the designs discussed above showed that both warp and weft yarns are primarily responsible for the creation of the depth of the pocket, thus limiting sheet caliper generation. Particularly, in single layer designs, the weft yarns show better control of pocket depth than the warp yarns. It is therefore observed that changing the profile of the weft yarns to a triangle or substantially triangular shaped cross-section instead of the conventional round yarns results in an increase of pocket depth, leading to higher sheet caliper and other desirable sheet characteristics.
The present invention provides an improved TAD fabric which exhibits favorable characteristics for the formation of tissue paper and related products.